An implant could be regarded as one of ideal biomaterials, if it could be prepared from a material which is safe with no toxicity and can be present in the living body for a while by performing its mechanical and physiological functions and objects during the healing period, but is gradually degraded and disintegrated thereafter to be absorbed in the living body and excreted therefrom via the metabolic pathway in the living body, so that the region where it was implanted could finally be replaced by the living body to reconstruct original conditions of the living body.
In recent years, artificial bones, artificial joints, artificial tooth roots, bone fillers and bone prosthetic as substitutes for biological bones and cartilages which are hard tissues, and materials for osteosynthesis for the purpose of fixing fractured cartilages or hard bones in respective regions have been produced making use of various metals, ceramics and polymers.
In the field of surgery such as orthopedic surgery, plastic surgery, thoracic surgery, oral surgery, brain surgery and the like, plates, screws, pins and the like made of metals or ceramics are used as materials for osteosynthesis with the aim of fixing and binding biological bones.
However, being excessively high in mechanical strength and elastic modulus in comparison with biological bones, the materials for osteosynthesis made of metals have problems of, for example, causing a phenomenon of reduced strength of peripheral bones due to stress protection after the treatment. Also, the materials for osteosynthesis made of ceramics have excellent hardness and rigidity but are brittle, so that they have a fatal defect that it is apt to be broken. With regard to polymers, attempts are being made to improve their strengths which are generally lower than those of bones.
On the other hand, bioactive bioceramics which can be bound directly to bones have been used in many cases by directly implanting into or contacting with the human body, for the purpose of recovering or improving biological functions.
Also, certain bioceramics which bind directly and strongly to the living body and are gradually replaced by the living body have been studied continuously because of their unknown possibilities.
However, though their rigidity and hardness are generally large, the use of bioceramics as implants has a limitation because of their brittle properties of being easily chipped or broken by the momentary impact force in comparison with the case of metals, so that development of a material which has toughness but with no brittleness has been required in this field.
On the other hand, several cases have been known about polymers which are used as implants into peripheral areas of hard tissues, such as a silicone resin to be used as a substitute for cartilages, a hardenable acrylic resin as dental cement and braided cords made of polyester or polypropylene fibers for use in ligaments.
However, inert and high strength ultra-high molecular weight polyethylene, polypropylene, polytetrafluoroethylene and the other polymers to be used as substitutes for hard tissues in the living body are significantly lack in strength as substitutes for biological bones when used as such. Accordingly, when they are used alone in substitution bones or screws, pins or plates for osteosynthetic purpose, they are apt to be damaged by their breakage, splitting or wrenching.
In consequence, attempts have been made to produce implants having high strength, making use of compounding techniques of plastics.
A carbon fiber reinforced plastic material is an example of such case, but it is not practical, because peeling occurs between fibers and matrix plastic when implanted in the living body for a prolonged period of time, and the delaminated carbon fibers are broken and stimulate the living body to cause inflammation.
In recent years, a polyortho ester (a polybutylene terephthalate-polyethylene glycol copolymer) which is considered to be able to bind to bones has been drawing attention of this field. However, since the strength of this polymer itself is lower than the biological bones, it has a problem still remains unsolved, i.e. whether or not its physical behavior after its binding to bones in the living body can conform with the biological bones.
Unlike the case of the just described polymer which is not absorbable in the living body, polylactic acid, polyglycolic acid, lactic acid-glycolic acid copolymer and polydioxanone which are degradable and absorbable in the living body have been put into practical use for a long time in the clinical field as absorbable sutures.
It has been considered for a long time that if such polymers used in sutures could be applied to materials for osteosynthesis, it would be possible to obtain a material for osteosynthesis having such excellent properties that reoperation after healing is not necessary and reconstructing of biological tissues is effected after absorption and disappearance of the polymer.
In view of such an expectation, studies have been conducted actively on the use of the aforementioned biodegradable and bioabsorbable polymers as materials for osteosynthesis.
For example, a self-reinforced type devices for osteosynthesis in which polyglycolic acid fibers are fused has been proposed (U.S. Pat. No. 4,968,317, specification) and used in the clinical field, but its disadvantages have also been pointed out that it is degraded quickly and, though it is rare, the fused fibers are delaminated and fine pieces of the delaminated fibers stimulate their surrounding region in the living body to cause inflammation.
Also, an unexamined published Japanese patent application (Kokai) No. 59-97654 discloses a method for the synthesis of a polylactic acid and a lactic acid-glycolic acid copolymer which can be used as biodegradable and bioabsorbable devices for osteosynthesis, but it shows only the polymerization product itself as an example of the material for osteosynthesis, does not describe about molding process of the material and shows no attempts to improve its strength to a degree similar to that of the human bones.
In consequence, with the aim of improving such strength, proposals have been made on a method for the production of pins for osteosynthesis in which a biodegradable and bioabsorbable polymer material such as of polylactic acid or the like containing a small amount of hydroxylapatite (to be referred simply to as HA hereinafter) is molded and then drawn and oriented in the longitudinal axis direction with heating (an unexamined published Japanese patent application (Kokai) No. 63-68155) and on a material for osteosynthesis which is obtained by drawing a molded product of a high molecular weight polylactic acid or lactic acid-glycolic acid copolymer having a viscosity average molecular weight of 200,000 or more after its melt molding (an unexamined published Japanese patent application (Kokai) No. 1-198553).
In the materials and pins for osteosynthesis obtained by these methods, the crystal axis (molecular axis) of the polymer materials is basically uni-axially oriented in the longitudinal axis direction, so that their bending strength and tensile strength in the longitudinal axis direction are improved. Particularly, the latter case of material for osteosynthesis having a viscosity average molecular weight of 200,000 or more after its melt molding is practical, because it shows high strength even in its low drawing ratio that fibrillation does not occur.
However, in the case of materials for osteosynthesis obtained by drawing basically only in the longitudinal axis direction, molecules (crystals) are oriented basically only in the longitudinal axis direction which is the molecular chain axis (crystal axis), so that the orientation anisotropy along the transverse direction as the right angle direction to the longitudinal axis direction becomes large, and the strength in the transverse direction therefore becomes weak relatively.
Also, according to the aforementioned unexamined published Japanese patent application (Kokai) No. 63-68155, a maximum bending strength of 162 MPa is barely obtained by drawing a mixture containing 5% by weight of HA. However, when it contains 20% by weight of HA, the bending strength is rather reduced to 74 MPa which is slightly higher than the pre-drawing value of 63 MPa.
However, since this maximum strength value does not fully exceed those of cortical bones, and the material becomes a porous heterogeneous article in which voids generated by the drawing are present in a large number between fillers and matrix polymer, it cannot be used for implants which require high strength such as substitutes for biological bones and materials for osteosynthesis.
In addition, the above published patent application also describes about a method for the production of plates in which powder of a biodegradable and bioabsorbable polymer material such as polylactic acid containing a small amount of HA is press-molded, but the plates are obtained merely by melt pressing of a mixture of HA and polylactic acid, and it does not describe a general idea of improving strength of the product taking its orientation into consideration.
In general, when biological bones are fixed using a material for osteosynthesis, forces in various directions are applied to the material for osteosynthesis. For example, in the case of a plate-shaped material for osteosynthesis, various forces such as bending force, tensile force, compressive force, tear force, shear force and the like are applied thereto, alone or in combination, and, in the case of a screw type material for osteosynthesis, a large torsional force is applied thereto when it is screwed into a biological bone and present in the living body, in addition to the above forces.
However, as described in the foregoing, in the case of a material for osteosynthesis obtained by drawing in the longitudinal axis direction, molecules are oriented only in the longitudinal axis direction which is the molecular chain axis [mechanical direction as the drawing axis], so that the molecular orientation anisotropy with the transverse direction as the right angle direction to the longitudinal axis direction becomes large.
Accordingly, the material is weak against tear strength from the longitudinal axis direction and shear breakage from the transverse direction and is also weak against torsional breakage which uses the longitudinal axis as the rotation axis. In consequence, when the just described tear force or shear force is applied to a material for osteosynthesis implanted in bones, the material for osteosynthesis will face a problem in that it is split or torn or generates shear fracture along a longitudinal axis direction relatively easily or a problem in that the material for osteosynthesis generates a torsional fracture when a torsional force is applied thereto using the longitudinal axis as the central axis of rotation like the case of a screw which is implanted in bones by loading a torque.
Such problems become more significant as the degree of fibrillation of the polymer material increases when its spherical structure reaches fibrous structure via a lamellar orientation by increased degree of drawing.
The present invention contemplates overcoming the aforementioned problems involved in the prior art, thereby providing biodegradable and bioabsorbable materials for osteosynthesis and implant which have less mechanical anisotropy and larger strength than a uni-axially oriented material obtained by longitudinal axial (uni-axial) drawing and in which their crystals are oriented basically not in the longitudinal axis direction but in parallel with a plurality of reference axes, as well as their production methods.